Sotah 38a
סוטה ל״ח א — ד"ה אָמַר אַבָּיֵי
Sotah 38a:15
Chachmei Bavel state that when there are two kohanim, the prayer leader calls “Kohanim,” but when there is only one he does not call “Kohen”; they also state that a kohen calls “Kohanim,” whereas a non-kohen does not, deriving both rules from “אמור להם.
אָמַר אַבָּיֵי: נָקְטִינַן, לִשְׁנַיִם קוֹרֵא ״כֹּהֲנִים״, וּלְאֶחָד אֵינוֹ קוֹרֵא ״כֹּהֵן״, שֶׁנֶּאֱמַר: ״אָמוֹר לָהֶם״ — לִשְׁנַיִם. וְאָמַר רַב חִסְדָּא: נָקְטִינַן, כֹּהֵן קוֹרֵא ״כֹּהֲנִים״, וְאֵין יִשְׂרָאֵל קוֹרֵא ״כֹּהֲנִים״, שֶׁנֶּאֱמַר: ״אָמוֹר לָהֶם״, אֲמִירָה
Abaye said: We have a tradition with regard to the prayer leader calling the priests to recite the Priestly Benediction: When there are two priests, he calls: Priests, but when there is one priest he does not call: Priest, as it is stated: “You shall say to them,” in plural, meaning to a minimum of two priests. And Rav Ḥisda said: We have a tradition that a priest calls: Priests, but an Israelite does not call: Priests, as it is stated: “You shall say to them,” which means that the saying